\m 



PHYLUM CHORDATA 



167 



stance somewhat resembling the bony cement or crusta petrosa 

 of the tooth. 



The skeleton is composed -of cartilage, with, in many cases, 

 deposition of calcareous matter in special places notably in the 

 jaws and the vertebral column. The 

 entire spinal column may be nearly 

 completely cartilaginous (Hexanchus 

 and Heptanchus), but usually the 

 centra are strengthened by radiating 

 or concentric lamellae of calcined 

 tissue ; or they may be completely 

 calcined. They are deeply amphi- 

 coelous, the remains of the notochord 

 persisting in the large inter-central 

 spaces. Intercalary pieces (Fig. 845, 

 Ic.) are interposed between both 

 superior and inferior arches. In the 

 Rays (Fig. 846) the anterior part of 

 the spinal column becomes converted 

 into a continuous solid cartilaginous 

 and calcified mass the anterior verte- 

 bral plate (a. v. p.). As in Fishes in 

 general, two regions are distinguish- 

 able in the spinal column the pre- 

 caudal and the caudal, the latter being 

 characterised by the possession of 

 inferior or haemal arches. In the pre- 

 caudal region short ribs may be developed, but these are sometimes 

 rudimentary or entirely absent. In the Sharks pterygiophores, 

 sometimes jointed, fused at their bases with the haemal spines, 



support the ventral 

 lobe of the caudal 

 fin, and the dorsal 

 lobe of the same fin 

 is supported by a 

 series of pterygio- 

 phores resembling 

 produced neural 

 spines, but only 

 secondarily related to 

 the spinal column, 

 and sometimes also 

 divided by joints. 

 The dorsal and ventral fins are sometimes supported by similar 

 pterygiophores ; but in many cases the cartilaginous supports 

 of these fins consist, in whole or in part, of expanded plates of carti- 

 lage. The marginal portions of the unpaired fins beyond the limits 



FIG. 844. Dermal denticles='of Centro- 

 phorus calceus, slightly magni- 

 fied. (From Gegenbaur's Comparative 

 Anatomy.) 



FIG. 845. Portion of the spinal column of Scymnus. Ic. 

 intercalary cartilages : Ob, neural arches ; WK, centra. 

 (From Wiedersheim's Vertebrata.) 



